Resistance thermometer



Patented Mar. 18, 1952 UNITED STATES PATENT OFFICE RESISTANCE THERMOMETER Oscar W. Roost, Foxboro, Mass., assignor to The Iioxboro Company, Foxboro, Mass., a corporation of Massachusetts Application September 9, 1949, Serial No. 114,769

2 Claims. 1

This invention relates to industrial thermometers and has particular reference to heat responsive bulbs in which temperature is determined by the effect of heat on the resistance of an electrical conductor.

In devices of the nature here considered, in some applications, a resistance type heat responsive unit may be extended directly into the medium to be temperature measured. In other applications a well may be extended into the medium to be temperature measured, a resist ance type heat responsive unit placed in the well, and measurement made of the reactions of the heat responsive unit to temperature changes in the medium.

Difficulties have been experienced in achiev-- ing eflicient transmission of heat from the medium into the heat responsive unit. Further difiiculties concern the dimensions and form of the heat responsive unit in that heretofore each unit has been made only for a single particular size and shape of well.

It is an object of this invention to provide a new and improved heat responsive unit with a maximum of accuracy and speed of response. It is a further object to provide such a unit which isreadily adaptable to a variety of well sizes and shapes, in particular, depths and diameters, and to provide improved means of transmitting the heat of the medium to the heat responsive unit. These and other objects will be apparent from the following specification and claims taken in the light of the accompanying drawings, in which:

Fig. 1 is a view in central longitudinal section of a structure embodying this invention;

Fig. 2 is a view showing in detail certain parts later to be described;

Fig. 3 is an enlarged detail view in central longitudinal section of the heat responsive unit; and

Fig. 4 is a transverse section on line 44 of Fig. 3.

Referring to the lower portion of Fig. 1, a well I extends into the process space of the medium to be temperature measured, and the heat from the medium is transferred by conduction through the well and into the heat responsive resistance unit 2 within the well in which, as will be later described, a small electric current fiow is maintained. Temperature changes in the medium cause electrical resistance changes in the resistance unit, and it is on the basis of the electric current flow change caused by this resistance change, that the temperature change in the medium is measured. The resistance unit is afiected by temperature change, whether it is an increase or a decrease.

The main elements of the assembly which contains the resistance unit comprise the well I, extending through and thread mounted in a wall 3 .of the container of the medium to be temperature measured, and carrying upon its upper end an adaptor coupling 4 which in turn carries a connection head 5 with a removable end cap 6 thereon.

The resistance unit 2 is carried on the lower end of an extension support tube I, from which it is readily removable. The support tube 1 is a plain, thin walled and lightweight tube formed of a low heat conductivity material. The resistance unit 2, which contains the heat responsive device of the electrical resistance type is relatively short and the readily separable connection between the unit 2 and its supporting extension tube 1 makes it possible to use a standard sized resistance unit for a variety of thermometers with wells of different size and lengths. In the past such units were not detachable, but were integral with their supporting parts and so would fit only one size and shape of Well. Now, the support tube may be replaced by a longer one for a longer well, or cut down to fit a shorter well, providing a large measure of interchangeability.

Referring to the lower portion of Fig. 3, high speed of response is achieved by rapid transmission of heat from the medium being temperature measured to the resistance unit 2. This is here accomplished by using in the unit a resistance wire bobbin 8 which is solid, and of a material of high thermal conductivity, and by providing a heat transfer relation (see Fig. 1) only between the bottom of the well and the lower end of the unit.

The resistance unit is pressed downwardly into the well I by an arrangement best shown in Fig. 2, which shows, in its central portion, the upper end of extension tube 1 in illustrative relation with the plate 9. The top of tube 1 is shown partially broken away, above where the tube passes through the plate 9.' Positioned over the end of tube 1 is the tiltable yoke Ill whose depending legs I I pass through the opening in plate 9 alongside tube 1, and are provided with turned-up feet I2 to hold compression spring I3 between them and the under side of plate 9. The upper portion of yoke ID has a central projection I 4 shaped to fit into the top open end of tube 1, so that when yoke I 0 is lifted and the projection I4 fitted into the top of the tube, the resulting compression of spring I3 applies downward force to tube 1. The yoke I0 is bent at its upper end to provide a finger hold whereby the yoke may be raised and tipped to permit the tube I and resistance unit 2 to be inserted or lifted out of the assembly.

-Also, the upper portion of the yoke ID hasan opening I5 therethrough which is used for mark-f ing support tubes, like tube 1, as an indication of tube 1 downward with the desired force. Whatever the depth of the well, this same force may be provided by compressing the spring l3 the same amount, that is, by lifting the yoke projection M the same distance above the plate 9. When a long tube is put in the well to be measured for length, the spring I3 is fully relaxed, and the yoke lifted until its feet I2 contact the bottom of the spring and the top of the spring contacts the bottom of the plate 9, without compressing the spring. With this arrangement, the yoke opening [5 is the same distance above the plate 9 as the top of the tube in Figure II. The tube to be cut is manually rotated and a marker extended through the opening to mark the cut off line on the tube.

The spring and yoke arrangementsabove described insures firm contact between the lower end of unit 2 and the bottom of the well I, thus producing a good heat transfer relationship. Such contacts are good when the lower end of the unit 2 and the inner surface of the bottom of the well i are made, or happen to be, so interfitting as to be substantially completely in surface to surface contact. However, when there is a possibility that the two surfaces are not complementary and the contact is only at spaced points or incomplete surface areas, another feature of this invention is to place in the bottom of the well, before unit 2 is inserted, a wad j 6, Figure I, of metal foil or other similar heat conductive and compressible material. The downward pressure on the unit aids in bedding down the foil IB so that surface-conforming contact is produced between the lower end of the unit and the inner bottom surface of the well.

For the detailed description of the construction of the resistance unit 2, reference is made to its sectioned enlargement in Figure III, which is devoted almost entirely to the showing of this unit. In the upper portion of the figure, however, the lower end of the'suppcrt tube T is shown, in-connected relation with the unit 2. This relation and the details of the connection will be described later herein.

In the lower third of Figure III, and centrally thereof, the bobbin 3 is shown. It is in the form of a one piece headed pin, of material of high heat conductivity, such as silver, with the head I! at the lower end and the cylindrical body 18 extending upward therefrom. Thehead ll forms the lower end of the resistance unit 2, and is the part thereof that makes contact with the bottom of the well, or, as in Figure I, with the foil iii.

For the purpose of producing a good surface to surface contact, the end surface of the head H is formed, as at H), in a convex rounded contour, or it may be formed as a downwardly extending truncated cone, to. fit well bottoms formed with a drill.

Returning to the lower portion of Figure III, and with further reference to the bobbin head II, it is provided with certain configurations, to be described, which not only assure thepresentation of the end surface I9 as the only lower end contact area of the resistance unit 2, but which alsocooperate in the assembly of theunit itself. The head II is generally cylindrical in form, with the side cut to form an annular groove 20 and at the same time forming two annular peripheries 2| and 22, of differentdiameters. The lower, or end periphery 2| has the lar er 4 of the two diameters, it being equal to the outside diameter of the unit 2 and that of the extension tube i. Since this is so, the head I! provides the only well contact area in the whole resistance unit. The manner in which the groove 20 and the smaller periphery 22 cooperate in the assembly of the resistance unit itself will be apparent from the further description herein.

The bobbin head H and the lower portion of the bobbin body l8 are partially cut away, inFigure III, to show that the head and body are integral and solid. From this arrangement and construction it will be seen that not only is excellent and eficient heat transferring contact made by the bobbin 8 with the well, but also, the solid bobbin provides a direct path for swift transfer of heat throughout the body of the bobbin.

The bobbin body 58, at its upper end, is connected to a generally cylindrical ce'ramic connection block 23, the outer diameter of which is somewhat greater than that of the bobbin body. The connection between the bobbin and the ceramic block 23 is accomplished by a threaded pin connection 26, and the upper portion of the bobbin body is cut away in Figure III to show this arrangement.

Wound about the bobbin body in a plurality of layers, is an electrical resistance wire 25, forming the resistance element of the unit. Heat variations reaching this winding from the medium being measured cause corresponding changes in the electrical resistance of the winding. This resistance wire is small and it is wound on the bobbin body between the ceramic block 23 and the bobbin head I? in non-inductive bi-filar fashion, with the length necessary to provide the desired amount of resistance. The wire, before it is wound, is covered with two servings of fibre glass, which act, when the wire is in place upon the bobbin, as electrical insulation between the winding turns and between the wire and the bobbin, and as each layer is wound, it is coated with silicone varnish to impregnate the fibre glass and fill the spaces between the winding turns. This varnish acts as a mechanical binder, gives better thermal conductivity between the bobbin and the wire, improves the electrical insulation characteristic of the fibre glass, and aids in excluding moisture from the winding. It has been found that three layers of .002 inch nickel resistance wire may be wound 111 .011 the bobbin without having excessive variation in thermal conduction between the bobbin and its surrounding wire.

The two free ends of the wire leave the bobbin at the inner or top end, one on each side of the bobbin. Only one of these leads is shown. 7

Referring now to the ceramic block 23, its diameter is reduced to form the peripheral recesses 26, which, in the finished unit; Sfirve to lock the parts in place. The block also has .longitidunal grooves 21, which permit the passage .of wires, as will be later described. An .end 'view of this ceramic block is shown in Figure IV.

Extending down through the support tube 1,

as shown in Figure III, and into the resistance unit 2, is a pair of electrical lead wires 28. These wires are separated at the top of the ceramic block 23, and continue down cnopposite sides of the block, in the longitudinal grooves 21. only one of these wires is shown below the top of the ceramic block but they both arein similar form and arrangement. In the lead wire shown, the normal round, insulation coveredform is main! tained while the wire is in the upper part of the groove 21. Then, from a point 29, the wire is stripped of insulation and flattened. A portion of the flattened wire is laterally and peripherally bent, as at 36, with respect to the ceramic block, into and cut of the lower of the peripheral recesses 25, to form an anchor for the lead wire as a safety device to prevent breakage strain, on the wire or any movement strain or deformation which would change the electrical resistance value of the wire 25. The lead wire is then con tinued beyond and below the ceramic block and is connected to one end of the resistance wire 25.

Still referring to Figure III, the assembly of the bobbin 8 and the ceramic block 23 is covered, with the exclusion of the bobbin head H, with a winding of glass cord 3!. This cord is impregnated, and the winding filled in, with silicone varnish as a moisture resistant mechanical holding agent. This cord aids in binding the winding 25 in place and also is wound in the ceramic block recesses 25 to bind the lead wires 28, and in particular the anchor portion 32.

Above the ceramic block 23, the lead wires 23 are encased in a fibre glass sleeve 32, and the whole assembly, again with the exception of the bobbin head [1, is covered with a layer of silicone rubber 33 to further seal the unit against moisture and to provide more electrical insulation.

The assembly just described has an outer, thin walled cylindrical casing 34 of low heat conduc tivity material. The lower end of this casin is secured to the bobbin head I! as the only direct connection between the bobbin and the casing. This connection is made by rolling the lower end of the casing into the annular groove 20 in the bobbin head [1. Because the bobbin material is soft, being silver, it conforms somewhat to the rolled-in contour to aid in sealing this connec tion against moisture and dirt. This connection is facilitated by the fact that while the diameter of the bobbin head periphery 2|, as previously mentioned, equals the outside diameter of the casing, the diameter of the bobbin head periphery 22 is less than the diameter of the periphery 2! by the amount of the thickness of the casing 34. The resistance unit 2 is preferably made with a quarter inch diameter, or less, in order to fit within the majority of wells in present use, and to provide the advantage of quick response to temperature change because of the small mass of the bobbin.

The casing 34 is so filled by the assembly that there are no appreciable air spaces in the unit. Above the ceramic block 23 the casing is reduced in diameter, forming a shoulder to receive the lower end of the tube 1, and is inserted in the lower end of the tube. A gripping arrangement is provided in the tube in the form of a lengthwise slot 35 and a construction of the tube in the area of the slot, suincient to bind the upper end of the casing, but still permitting ready disassembly. The casing 34, as it extends above the ceramic block 23, is filled with silicone rubber in continuation of the covering layer 33 of the same material, and this completes the thermal isolation of the resistance wire 25 from the tube '1.

Referring to the upper portion of Figure I, the lead wires 28, as extended from the top of Figure III, emerge from the upper end of the support tube 1. Adjacent this point the support plate 36 is located and is used to mount whatever electrical units are desired to be contained in the head 5 as a part of a circuit which includes the resistance winding 25 of the unit 2. Since this invention is not concerned specifically with such electrical units, the leads 23 which, as in Figure 1E, extend from the resistance unit 2 where they are connected to the winding 25, up through the tube 1, are simply shown in Figure I as extending outward from the top of the tube 1, through connection points and through the wall of the connection head 5 by way of the opening 31 to a suitable electrical power source, not shown, for maintaining the small, electric flow in the winding 25, this current being changed as the resistance of the winding wire is changed by temperature variation.

Having described my invention, I claim:

. l. A temperature sensitive unit comprising a solid, integrally headed bobbin, a ceramic connector block mounted on the body end of said bobbin, a resistance wire winding on said bobbin between said connector block and the head of said bobbin, electrical lead wires connected to said winding, a binding cord winding over said wire winding and over said block and lead wires, a casing over said cord winding and secured to the head of said bobbin, and a body of insulation between said casing and said cord winding, leaving no air space therebetween and extending over the free end of said ceramic block in covering relation therewith.

2. In a temperature sensing arrangement for use as a part of an industrial thermometer, a well to be extended into a process space, and an assembly to be held in said well in heat transfer relation with the bottom only of said well and free of side wall contact therewith, said assembly comprising a support tube and a relatively small readily detachable sealed unit mounted on said tube and lying wholly within said well, said sealed unit comprising a solid, integrally headed bobbin of heat conductive material, a ceramic connector block mounted on the body end of said bobbin, an electrical resistance wire winding on said bobbin between said connector block and the head of said bobbin, lead wires connected to said winding and extending along said ceramic block and through said tube, a fibre glass covering on said resistance wire, a varnish coating over and between the coils of said resistance winding, a varnished glass fibre binding cord winding over said wire winding and over said ceramic block and lead wires, a tubular casing over said cord winding, having one end secured to the head of said bobbin and the other end effecting said readily detachable mounting with said support tube, and a body of rubber-like insulation between said casing and said cord winding and extending over the free end of said ceramic block in covering relation therewith, whereby no air space is left between said bobbin or said ceramic block, and said casing.

OSCAR W. ROOST.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,444,410 Keinath June 29, 1948 2,476,099 Knudsen July 12, 1949 

